Hydraulic Pumps and Hydraulic Motors

If increasing the size, probably not, as the pipe friction losses will be lower.

If decreasing the size, then the pipe friction losses will increase. So the power available at the motor may decrease. By how much it is hard to say.

Thank you very much for your kind reply

Yes, the motor power will decrease, though maybe not by much. The smaller lines will decrease the pressure delivered to the motor and will also increase the downstream pressure. Thus the lesser differential pressure across the motor might realistically decrease by about 1% to 10% (or worse), depending on how drastically the line size is reduced.

Assuming that no other hydraulic load is in the circuit (no other motors, no cylinders, etc.): no, it won't affect the motor's power or performance. Those two factors depend directly on the load that's put on the motor.

What will happen is that more pressure will be needed to drive hydraulic fluid inside the pipe. The hydraulic pump will thus have to work harder to push fluid through the pipe.

The result: smaller pipe means more power consumption; and in extreme cases, the pump will be too small to handle the smaller pipe, so the pump's motor protection will break the pump's power feed.

Cheers! DZ

Changing the size of the lines to the motor from the pump will cause a differance. But if you can deliver the same volume of fluid your speed should remain the same. However you might see a small increase in pressure due to line restriction and this would be normal if the hp was to remain the same. If you don't see an increase in pressure I would say you have a bit of a line loss issue and have lost power.

Is there a good reason to change the lines?

check the pressure in motor port line during running. no change in pressure drop from pump to motor at same speed means no problem

The real answer depends on how undersized you make the lines, what pressure and what flow you are running.

Properly sized fluid conductors transmit power with minimal line losses. When I design hydraulic systems, the line size is definitely a critical component to a properly operating system.

The cross sectional area of your fluid conductor should be sufficient to keep your fluid velocity below 15ft/sec (pressure dependant) on pressure lines. If your fluid velocity exceeds 15ft/sec you can cause turbulence and erosion in your lines which can cause you problems in the long term, and reduce your system efficiency. In addition, by running your fluid velocity above proper design limits you reduce the actual power available to the motor from the pump due to line losses from the friction in the conductor. So, Yes, you will reduce the true power available at your load by undersizing the lines. How much losses are acceptable, is entirely objective. I can run the numbers to show you quantitatively exactly how much power you will reduce at your load by reducing your line size. In most simple cases, the line losses are so small in comparison to the total system power it's considered negligible.

Line sizes are especially critical on the pump intake and return lines. Return lines should not exceed 10ft/sec, and pump suction lines should be designed so the fluid never exceeds 3ft/sec. Improper pump intake line sizes and designs account for an alarming rate of pump failures in the hydraulics industry. Undersized intake lines will cause the pump to cavitate, reducing the efficiency and life of the pump drastically.